Wireless communication systems typically employ frequency synthesizer technology in both the receive path circuitry and the transmit path circuitry. One significant application for frequency synthesizers is in mobile phone systems which transmit and receive on many different frequency channels. For example, the United States and Europe have adopted mobile phone standards with communication centered in two frequency bands at about 900 MHz and 2300 MHz. Each of these bands has a large number of dedicated transmit frequency channels and receive frequency channels. A frequency synthesizer enables a wireless unit to tune among the many channels of such bands as needed.
Frequency synthesizers typically employ a phase locked-loop (PLL) together with divider and phase detector circuitry to enable a wireless unit to switch from channel to channel. PLL circuits include voltage controlled oscillators (VCOs) which are controlled via feedback and an error signal to produce the desired output frequency (fout). In more detail, the output frequency (fout) may be made programmable by utilizing an output feedback divider (÷N) and a reference divider (÷R) for an input reference frequency (fref). The output frequency produced is a function of the values selected for “N” and “R” in the divider circuits, such that (fout)=N(fref/R). The PLL circuitry typically utilizes a phase detector to monitor phase differences (Δφ) between the divided reference frequency (fref/R) and the divided output frequency (fout/N) to drive a charge pump. The charge pump delivers packets of charge proportional to the phase difference (Δφ) to a loop filter. The loop filter outputs a voltage that is connected to the VCO to control its output frequency. The feedback loop thus formed attempts to drive the phase difference (Δφ, which acts as an error signal) to zero (or at least to a constant value) in order to provide a stable and programmable output frequency (fout).
The frequency synthesizer described above employs a single path PLL. Frequency synthesizers are also available which employ two PLL paths, namely dual path PLL frequency synthesizers. In that approach the PLL includes a direct path loop filter and an integrating path loop filter which operate in continuous time. Unfortunately, the direct and integrating paths employed in this dual path, continuous time PLL approach tend to be difficult to match. One cause of this difficulty is that each of the dual paths can exhibit a different amount of temperature drift. Moreover, dual path PLLs are unfortunately prone to spurious outputs.
What is needed is a frequency synthesizer that overcomes the above described problems.